They spent roughly a thousand days and nights together, from the rainy October morning they left the falls of the Ohio until they finally pulled their canoes out of the Mississippi three years later in St. Louis. They slept in impossibly close quarters, often sharing the same buffalo-skin teepee with an Indian woman, a French-Canadian interpreter and their baby. They, and several enlisted men, kept journals whose published throw weight equals 13 volumes, 30 lbs., 18 in. of bookshelf and approximately 1 million words. All that evidence notwithstanding, the more we learn about the two captains who gave their names to...